Techniques of preparing liquid crystal light modulating materials by phase separation, as well as the advantages offered by such techniques and the materials prepared thereby, are discussed in U.S. Pat. Nos., 4,671,618, 4,673,255, 4,685,771 and 4,688,900, the disclosures of which are hereby incorporated by reference.
Switchable electro-optic display cells exhibiting a varying gray scale between substantial transparency and substantial opacity are proposed in U.S. Pat. No. 4,411,495 and in Applied Physics Letters, Vol. 40, p. 22 (1982). Such display cells employ anisotropic suspensions imbibed into a porous cellulosic material sandwiched between transparent electrodes; in the '495 patent, the anisotropic suspension comprises particulates, whereas in the Applied Physics Letters article, the anisotropic suspension comprises nematic liquid crystal with positive dielectric anisotropy. The operation of these display cells depends upon matching or mismatching the index of refraction of the isotropic porous material to the index of refraction of the anisotropic liquid crystal or anisotropic particulate suspension. Transparency is achieved when the index of refraction of the porous material matches the index of refraction presented to incident light by the material within the pores. The orientation of the liquid crystal or particle within the pores is generally random in the absence of an applied field and in the presence of a field generally parallel to the direction of the field. Thus, in the absence of an applied field, the material in the pores presents an overall index of refraction n, equal to a value between the ordinary n.sub.o and extraordinary n.sub.e indices of refraction. Transparency in the field-off condition takes place when the index of refraction of the porous material is matched to the overall index of refraction n of the material in the pores. Subsequent application of a field results in the liquid crystals or particles tending to align in the direction of the field, thus changing the index of refraction presented to probing light, with resultant scattering or opacity.
A display cell that operates in a manner opposite to that just described is obtained where the index of refraction of the porous material is matched to the ordinary index of refraction n.sub.o of the liquid crystal or particle. In this case, application of a field results in the liquid crystal or particle aligning with the field and presenting its ordinary index of refraction n.sub.o to incident light so that it detects no mismatch between the indices and is transmitted. Subsequent removal of the field results in a return to a random state wherein an overall refractive index n, not equal to that of the porous material, is presented to incoming light which is then scattered with the cell appearing opaque. Between the extrema of maximum transparency and maximum opacity, a gray scale is achievable as a function of the voltage applied across the cell.
Each of the above two modes of operation displays a different profile at which the cell is transparent to a viewer. In the case where the index of refraction of the porous material is matched to the overall index of refraction n of the randomly aligned liquid crystal or anisotropic suspension (field-off transparency) the device appears transparent to a viewer from essentially all directions of view due to the fact that the overall index of refraction n is essentially isotropic, i.e., the same for all viewing angles. But in the case where the index of refraction of the porous material is matched to the ordinary index of refraction n.sub.o of the aligned liquid crystal (field-on transparency), the device appears most transparent viewed directly in the direction of the field (usually orthogonal to the cell), with the transparency falling off at increasingly oblique viewing angles due to the fact that the index of refraction in the aligned state is anisotropic: the further from the orthogonal the viewing angle, the greater the mismatch between the perceived indices until an essentially opaque appearance is detected by the viewer at an oblique enough angle.
While the described display cells make it possible to some extent to regulate the angle of view through a cell, these and other types of liquid crystal cells fabricated by the mechanical entrapment of liquid crystals in porous sheets or into capsules of polyvinyl alcohol or the like have a number of drawbacks. One of the drawbacks is the essentially unalterable index of refraction of the various entrapment materials: matching or mismatching is a trial and error process.
The techniques of the present invention make possible the frabrication of light modulating liquid crystal materials having "customized" viewing angles in the sense that the angle at which a window or display fabricated with such material is transparent or visible is predetermined by a deliberate matching or mismatching of the index of refraction of the synthetic resin matrix with a selected index of refraction of the anisotropic liquid crystal. Specific devices resulting from this invention include windows for head-up displays, sunscreens, windows with a built-in "venetian blind" feature and, in general, devices wherein the direction of transmission of incident light is restricted or modified as compared to previously described devices.